1. Field
The present invention relates generally to data communication, and more specifically to a novel and improved method and apparatus for processing data in a multiple-input multiple-output (MIMO) communication system utilizing channel state information to provide improved system performance.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), orthogonal frequency division multiplex (OFDM), or some other multiplexing techniques. OFDM systems may provide high performance for some channel environments.
In a terrestrial communication system (e.g., a cellular system, a broadcast system, a multi-channel multi-point distribution system (MMDS), and others), an RF modulated signal from a transmitter unit may reach a receiver unit via a number of transmission paths. The characteristics of the transmission paths typically vary over time due to a number of factors such as fading and multipath.
To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used for data transmission. If the transmission paths between the transmit and receive antennas are linearly independent (i.e., a transmission on one path is not formed as a linear combination of the transmissions on other paths), which is generally true to at least an extent, then the likelihood of correctly receiving a data transmission increases as the number of antennas increases. Generally, diversity increases and performance improves as the number of transmit and receive antennas increases.
A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NC independent channels, with NCxe2x89xa6min{NT, NR}. Each of the NC independent channels is also referred to as a spatial subchannel of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
There is therefore a need in the art for techniques to process a data transmission at both the transmitter and receiver units to take advantage of the additional dimensionalities created by a MIMO system to provide improved system performance.
Aspects of the invention provide techniques to process the received signals at a receiver unit in a multiple-input multiple-output (MIMO) system to recover the transmitted data, and to adjust the data processing at a transmitter unit based on estimated characteristics of a MIMO channel used for data transmission. In an aspect, a xe2x80x9csuccessive cancellationxe2x80x9d receiver processing technique (described below) is used to process the received signals. In another aspect, the channel characteristics are estimated and reported back to the transmitter system and used to adjust (i.e., adapt) the processing (e.g., coding, modulation, and so on) of data prior to transmission. Using a combination of the successive cancellation receiver processing technique and adaptive transmitter processing technique, high performance may be achieved for the MIMO system.
A specific embodiment of the invention provides a method for sending data from a transmitter unit to a receiver unit in a MIMO communication system. In accordance with the method, at the receiver unit, a number of signals are initially received via a number of receive antennas, with each received signal comprising a combination of one or more signals transmitted from the transmitter unit. The received signals are processed in accordance with a successive cancellation receiver processing technique to provide a number of decoded data streams, which are estimates of the data streams transmitted from the transmitter unit. Channel state information (CSI) indicative of characteristics of a MIMO channel used to transmit the data steams are also determined and transmitted back to the transmitter unit. At the transmitter unit, each data stream is adaptively processed prior to transmission over the MIMO channel in accordance with the received CSI.
The successive cancellation receiver processing scheme typically performs a number of iterations to provide the decoded data streams, one iteration for each decoded data stream. For each iteration, a number of input signals for the iteration are processed in accordance with a particular linear or non-linear processing scheme to provide one or more symbol streams. One of the symbol streams is then selected and processed to provide a decoded data stream. A number of modified signals are also derived based on the input signals, with the modified signals having components due to the decoded data stream approximately removed (i.e., canceled). The input signals for a first iteration are the received signals and the input signals for each subsequent iteration are the modified signals from a preceding iteration.
Various linear and non-linear processing schemes may be used to process the input signals. For a non-dispersive channel (i.e., with flat fading), a channel correlation matrix inversion (CCMI) technique, a minimum mean square error (MMSE) technique, or some other techniques may be used. And for a time-dispersive channel (i.e., with frequency selective fading), an MMSE linear equalizer (MMSE-LE), a decision feedback equalizer (DFE), a maximum-likelihood sequence estimator (MLSE), or some other techniques may be used.
The available CSI may include, for example, the signal-to-noise-plus-interference (SNR) of each transmission channel to be used for data transmission. At the transmitter unit, the data for each transmission channel may be coded based on the CSI associated with that channel, and the coded data for each transmission channel may further be modulated in accordance with a modulation scheme selected based on the CSI.
The invention further provides methods, systems, and apparatus that implement various aspects, embodiments, and features of the invention, as described in further detail below.